They are different tools.
Photoshop elements is for pixel editing. Aperture is an image development and cataloguing program, which you may then send to a pixel editor (Photoshop, Elements, GIMP etc) later for further editing. That's not to say elements can't do what aperture does, just it's aimed at single images not fast work over many images.
Personally I wouldn't be buying aperture at the moment. It's overdue an update and it's not clear if apple has any interest in releasing one. Many key Aperture users are starting to switch to Adobes Lightroom (which I just upgraded to version 4 and think is excellent)